Ringorang Challenges are housed in a Series.  It is possible to have a single Challenge in a Series.  However, many Sponsors run multiple Challenges represented in a Series.  

When a user joins a Challenge, they are automatically joined to the Series of which that Challenge is a part.  When other Challenges are launched within that same Series, the user is automatically joined to them as well.  

If a sponsor wants to enroll a Challenge with only new users and have no previous users automatically joined to it, then the sponsor must set up that Challenge in a different Series.

Users can read information about a given Series in the Series Details screen that invites the user to join or unsubscribe.

Related FAQ:  What is a Challenge?